UK Postage bought through amazon being posted late?

I buy all postage through Amazon and am sure to post with the labels produced the next day after they’re bought, as each label say to post by then. My post office acted surprised when I told them that, they seem to think it doesnt matter when they’re posted, they dont expire, like standard stamps, but I’m unconvinced. Anyone know for a fact either way?

This is the official line.

“ 4 Each Postage Label will expire at 11.59 p.m. on the working day after its purchase. A Postage Label will expire even if it has been downloaded and/or printed. This means that an Item must be posted by no later than the “Post by” date stated on the Postage Label.”

Amazon/Royal Mail Online Postage Terms and Conditions